Structure of heart, Function of its Chambers and valves

Heart is a muscular organ situated in the middle of the chest slightly to the left. It is hollow and conical shaped structure which is enclosed within a pericardium, the peritoneal counterpart for heart.

It is placed obliquely, behind the body of sternum and costal cartilages, one third of it being to right, and rest two third lying left of median plane. It measures 12*9cm, weighs 300g in males and 250g in females.

Courtesy: Springer.

External structure

There are four chambers in a human heart. Each right and left halves contain one atria and one ventricle each. Atria are the chambers for receiving blood and the ventricles pump out the blood to respective sectors.

Atria lies above and behind the ventricles. On surface of heart, ventricles are separated from atria by atrioventriculargrooves, and the ventricles are separated from each other by inter-ventricular grooves.

Circulation through body:

Human body consists of two types of circulation.

Systemic Circulation: This circulation starts from the left ventricle, exists through the Aorta, and supplies the whole body, both superior and inferior parts. The body tissues get oxygenated blood through this circulation. This ends in right atrium, the blood comes back from the upper part of body via Superior Venacava, and the inferior part of body via Inferior Venacava.

Pulmonary Circulation: This circulation starts from the right ventricle, exists heart via Pulmonary Artery which immediately divides into two branches to go into two lungs where it gets oxygenated and returns to left atria via four Pulmonary Vein.

We should remember that Pulmonary Artery and Pulmonary veins are exceptional blood vessels, where an artery carries deoxygenated blood and vein carries oxygenated blood.

Valves of Heart:

The heart is supplied with a series of valves, each having different functions. The main function of valves is to check the backflow of blood.

Classification:

The valves present in heart are of two categories:

Auriculo-ventricular valves: They are two in number namely

Tricuspid valves: Present between the right atria and right ventricle, it is named Tricuspid because of its three cusps

Bicuspid valves: Present between the left atria and left ventricle, it is called Bicuspid because of its two cusps. It is also named as Mitral Valve.

Semilunar Valves: Present between the great vessels i.e Aorta and Pulmonary Artery, and Ventricles, they are named such because of their structure. They are two in number:

Aortic valve: Present in between left ventricle and aorta.

Pulmonary Valve: Present in between right ventricle and Pulmonary Artery.

Anatomy:

ATRIOVENTRICULAR VALVES

Both Tricuspid and Bicuspid valves are made up of following components:

Cusps are attached to a fibrous ring.

The cusps are flat and projected into the ventricular cavity. Each cusp has an attached and free margin, an atrial and a ventricular surface. The atrial surfaces are smooth. The attachments of chordae tendinae make the free margins and ventricular surfaces rough and irregular. The valves are remain closed during ventricular systole by apposition of atrial surfaces near to the serrated margins.

The chordae tendinae join the free margins and ventricular surfaces of the cusps to the apices of the papillary muscles. Chordae tendinae prevent the eversion of the free margins and limit the amount of ballooning of the cusps towards the cavity of atrium.

The atrioventricular valves are kept competent by the active contraction of the papillary muscles. During the ventricular systole, these papillary muscles pull the chordae tendinae. Each papillary muscle is connected to the contiguous halves of the two cusps.

Blood vessels are seens only in the fibrous ring and in the basal one third of the cusps. Nutrition to the central two third of the cusps are directly derived from the blood in cavity of the heart.

The Tricuspid valve has three cusps. It can admit the tips of three fingers. The three cusps are namely anterior, posterior or inferior, and septal. These lies against three walls of ventricle. Of the three papillary muscles, anterior muscle is largest, inferior one is smaller and irregular, and septal muscle is denoted by a few small muscular elevations.

Mitral or Bicuspid valve has two valves- a large anterior called aortic cusp, and a smaller posterior cusp. It can admit tip of two fingers. The anterior cusp lies between Mitral and Aortic orifices. The Mitral cusps are relatively smaller and thicker than Tricuspid cusps.

SEMI-LUNAR VALVES

The aortic and pulmonary valves are referred as Semi-lunar valves due to their shape. Both the valves are similar to each other.

Each valve consists of three cusps which are attached directly to the vessel wall. There are no fibrous rings like atrioventricular valves. The cusps form small pockets, with their mouth directly away from ventricular cavity. The free margins of each cusps contain a central fibrous nodule, from each side of which a thin smooth margin, the lunule extends up to the base of each cusps. Each valve is closed during ventricular diastole, preventing the back flow of blood into ventricles, when the cusps bulge towards ventricular cavity.

Opposite to the cusps, the vessel walls are slightly dilated to form aortic and pulmonary sinuses. Coronary arteries arise from anterior and left posterior aortic sinuses.

Chambers of Heart :

Human heart is a four chambered organ where there is complete separation of oxygenated and deoxygenated blood. It has two atria and two ventricles.

Right Atrium: Right atrium is the right upper chamber of heart. It receives venous blood from systemic circulation, i.e. whole body, and pumps it into right ventricle via Tricuspid opening.

Along the right border of the atrium, there is a shallow groove, which passes from superior Venacava to inferior Venacava. It is called Sulcus Terminalis, produced by Crista Terminalis. The upper part of Sulcus Terminalis contains the Sino Atrial Node or SA NODE, the pacemaker of heart.

The right atrium is more or less vertically elongated structure receiving the Superior Venacava from upper end and Inferior Venacava from lower end.

Right Ventricle: Right Ventricle is a triangular chamber which receives blood from right atrium, and pumps it to lungs through Pulmonary trunk.

Interior of it shows two orifices – right atrioventricular or Tricuspid orifice guarded by Tricuspid valve and pulmonary orifice guarded by pulmonary valve.

The septomarginal trabecula or moderator band is a muscular ridge extending from ventricular septum to base of anterior papillary muscle. It contains right branch of Atrio Ventricular Node or AV NODE.

Wall of right ventricle is thinner than that of left ventricle. Ratio of thickness is 1:3.

Left Atrium: This is a quadrangular chamber located posterior to right atrium. It receives oxygenated blood via four branches of pulmonary veins, and pumps the same into left ventricle via Left Atrioventricular aperture or Mitral orifice guarded by valve of same name.

Two Pulmonary veins open into left atrium on each side of posterior wall.

In the embryonic phase, the right atrium and left atrium remains attached through a opening in the common atrial septum called FORAMEN OVALE. Later after birth, it closes, but an impression remains in both the atria. In right atrium it is called Fosaa Ovalis, and in left atrium, it is called Fossa Lunata.

Left Ventricle: Left Ventricle receives oxygenated blood from left atrium via mitral orifice and pumps the same to Aorta via Aortic valves.

Its interior shows two orifices –left atrioventricular or Mitral orifice guarded by Mitral Valves and the Aortic Orifice guarded by the Aortic Valve.

It is the thickest chamber of heart, about three times thicker than right ventricle, and almost circular in cross section.